From 1991 to 1997, for example, Virgin Publishing made over 50 New Adventures novels. Since then, the BBC has made with the novels themselves. Also, from 1979 to now, there's been Doctor Who magazine, dutifully writing several pages of comics adventures per month. Some of these books and comics are dreadful. Many are awesome. Probably. I've barely scratched the surface of what's out there.
But today I sing in praise of the Audio Adventures. Thankfully, you can't hear me from there, 'cause it's kind of an improvised singing. Big Finish Audio has been making 'Doctor Who' audio plays since the nineties and with generally much better writing than the TV series had at times.
The 5th through 8th Doctors have been adding to the breadth and depth of their onscreen journeys, whose only budgetary limits are YOUR imagination.
In my humble (but correct) opinion, the best BFA adventures are:
FOR THE 5TH DOCTOR: 'Circular Time': a tale of his life in four parts, not coincidentally by my beloved Paul Cornell. My ranking lists follow it closely with Marc Platt's 'Spare Parts': a tale of creeping global death by Cybernization, which earned Platt onscreen credit for the Cybermen's return in NuWho. My LEAST favorite 5th Doc story was 'Creature of Beauty': with some uninspired mutant business. And Nyssa is a treat as a companion on her own, which the TV series didn't have enough of.
FOR THE 6TH DOCTOR: 'The One Doctor': a hilarious showcase of six's ego coupled with his towering talent. My runner-up is 'The Maltese Penguin' for featuring the penguin-shaped private eye Frobisher (a former comic strip companion). Yes, a talking penguin. No, I don't think that's stupid. I think that's exactly what comics, novels and radio is FOR- being as outlandish as you like. Still, 'Catch-1782' was my least favorite, for having rather too much Melanie and making rather too little sense.
FOR THE 7TH DOCTOR: 'Bang-Bang-A-Boom': It's another funny one, a catastrophic combination of Star Trek: Deep Space Nine and the Eurovision song contest. When Seven meets the busty, operatic, space amazon I think I may have had kittens. From laughing. 'Shadow of the Scourge' was also really good. (Cornell, the best two companions of the decade, and what look like N'Grath from 'Babylon 5' attacking them on the cover. 'Frozen Time' was not so great, Maryam d'Abo notwithstanding.
FOR THE 8TH DOCTOR: Second to 'Zagreus' which has everybody, I ranked 'Shada', the retelling of Douglas Adams' unfinished story. It has Romana and K-9 in it, and Professor Chronotis is a splendid fellow. I didn't especially enjoy 'Something Inside', though, India Fisher notwithstanding.
India is a great example of all the companions you'd miss out on if you only think TV is "real" Doc Who. As Edwardian adventuress Charley Pollard she beat TV's River Song to the 'met the Doctor out of order' punch. And the 'keeping big secrets from him' punch.
And beating River Song to everything else: Bernice Surpise Summerfield. 'Benny' was the companion created by Paul Cornell to supplement/replace Ace in the New Adventures novels. Feisty archeologist, thirty years old in 2570, quick wit, liked the drink, liked the lads, and has become more insanely popular in novels and audio adventures than any TV companion. Sarah Jane? Audio adventures for several seasons. Very respectable (wish they hadn't ended with a cliffhanger...). Benny? STILL GOING. At 11 audio seasons, Benny has a shot not only at longest-running Doctor Who companion, but at longest-running spin-off character in western culture. Suck it, Frasier Crane!
Lisa Bowerman, the voice and essence of Bernice, had a role alongside Ace in 'Survival' as Karra the Cheetah Person, and as Benny she travelled with Ace and the Seventh Doctor for a long time in these off screen 'realities'. And they're ALL as real as you like. What you lose in visuals is gained in NOT having visuals: there's no limits. Not in violence, nor mature content, nor what-have-you. Nor WHO have you, in Benny's case...
I haven't scratched Benny's surface yet, either. (Unlike the EIGHTH DOCTOR!!! WHOOOO! No, seriously, Bernice and 8, sitting in a tree, etc. and so on, they totally had sex.)
My favorite Benny solo story so far was the pantomime extra-dimensional romp 'Oh No It Isn't' with Nicholas 'The Brigadier' Courtney stealing the show as Wolsey the Cat. You CAN'T make stuff like this on TV.
Not in my lifetime, anyway, I'll wager.